1. Field of the Invention
This invention relates to steering mechanisms for vehicles. More particularly, it relates to an apparatus and method for steering a vehicle to follow a track or guideway.
2. Description of the Prior Art
Vehicles adapted to travel on a guideway having a guide rail extending parallel to the desired course of the vehicle typically employ a guide frame with guide wheels to track the path of the guide rail and to turn steerable drive tires on the vehicle to follow the guide rail. However, when the vehicle travels along a curve on the guideway, it is not sufficient merely to align the steerable drive tires exactly along the curvature of the guideway. If this is done, the centrifugal forces and the scrub forces due to drive tire misalignment associated with turning the vehicle will be taken by the guide wheels. This phenomenon may be addressed relatively easily for a vehicle traveling in only one direction by adjusting the orientation of the steerable drive tires to compensate the centrifugal force by introducing lateral forces on the drive tires. However, as will be explained further, the compensation of the front steerable drive tire is different than the compensation of the back steerable drive tire. A steering apparatus modified for travel in one direction would not operate efficiently for travel in the opposite direction.
As a result, without any adjustment of the front and rear steerable drive tires, the guide frame cannot deviate from the predetermined curvature of the guide rail, and the guide frame forces the vehicle to travel along the predetermined path of the guide rail.
Furthermore, without any adjustment of the front and rear steerable drive tires, the guide frame must be designed not only to turn the steerable drive tires but also to tolerate centrifugal forces necessary to force the vehicle to turn. As a result, the forces transmitted upon the guide rail through the guide frame are excessive and larger structural elements are required in the fabrication of these parts.
To relieve these excessive forces on the guide frame, it is necessary to adjust the direction of the steerable drive tires to track parallel to the guideway without excessive forces upon the wheels. To do this, the steerable drive tires must be turned beyond the tangential orientation of the rail curvature.
U.S. Pat. No. 4,089,272 teaches a vehicle guidance apparatus which maintains the drive tires in a position parallel to the guide rail. As a result, all of the lateral forces produced when the vehicle turns are transmitted to the guide wheels.
U.S. Pat. No. 4,299,172 entitled xe2x80x9cSteering Sense Reversing Mechanism for Guided Vehiclesxe2x80x9d illustrates a passive steering system with guide frames extending from the front and rear ends of the vehicle to orient the steerable drive tires properly relative to a guideway. This passive system compensates for the forces tending to laterally displace the vehicle whether the vehicle is traveling in the forward or rearward direction.
A design is needed which may easily position both the front and rear steerable drive tires of a vehicle following a guide rail in the proper orientation to account for the radial drift associated with steerable drive tires when the vehicle turns.
The invention is directed to a steering apparatus for a vehicle adapted to travel on a guideway having at least one guide rail extending parallel to the desired course of the vehicle. The vehicle rests upon a support frame secured to a wheel assembly. The wheel assembly has a guide frame adapted to pivot about a guide frame pivot point such that a guide frame longitudinal axis remains parallel to the guide rail when the guide frame is following a straight portion of the guide rail and remains tangential to the guide rail when the guide frame is following a. curved portion of the guide rail. The wheel assembly further includes a first steerable drive tire hub connected to a first axle which is supported by the support frame and pivotable about a pivot for turning the tire. The steering apparatus comprises a steering linkage connecting the guide frame to the first steerable drive tire hub of the wheel assembly for turning the first steerable drive tire parallel to the guide rail when the guide frame is following a portion of the guide rail that is straight and for turning the first steerable drive tire tangential to the guide rail when the guide frame is following a portion of the guide rail that is curved, thereby defining a neutral steering angle between a longitudinal axis extending through the tire and a central longitudinal axis extending through the vehicle body. The apparatus is further comprised of an actuator for amplifying or reducing the motion of the steering linkage when the guide frame pivots along a curved guide rail by introducing a compensation angle to the neutral steering angle thereby producing an adjusted steering angle to account for a slip angle of the steerable drive tire.
The invention is also directed toward a vehicle having the subject steering apparatus.